The present invention relates to an optical disc and a method for recording data on the optical disc at a high density, and reproducing information dependent on the data, and a system for carrying out the method.
There has been proposed various ways to increase the density of pits formed on the optical disc. For example, the distance between the pits on the track is decreased, or the diameter of a laser beam which records and reads information is constricted.
Another method is to decrease the pitches between the tracks. However, if the diameter of a beam spot formed by the laser beam on the optical disc is not decreased, data recorded on the adjacent tracks are also reproduced, so that crosstalk occurs. In order to reproduce the correct information, there has been proposed a system for eliminating the crosstalk. Namely, three adjacent tracks are simultaneously read with three light beams. Signals produced from the side tracks are subtracted from a signal produced from the central track, thus cancelling the crosstalk.
More particularly, Japanese Patent Application 3-40225 Laid-Open discloses a system having a frequency-variable filter. The frequency is set at a value where the crosstalk is most effectively eliminated.
There has also been proposed a system wherein a cancelling coefficient is obtained so that correlations between the signals produced from the adjacent tracks are eliminated. For details, refer to pages 98 to 99 of Nikkei Electronics, Nikkei BP, No. 495, Mar. 19, 1991.
However, in the above-described reproducing systems, a signal corresponding to the crosstalk is produced from the adjacent tracks to cancel the crosstalk. Hence the tracks must be irradiated with a plurality of beams. Moreover, the frequency-variable filter is required in the first example. In the second example, a circuit for producing a reference signal which corresponds to a preamble signal recorded on the optical disc, and a detecting circuit for detecting the difference between the reference signal and the preamble signal are necessary. Thus, the systems become complicated in construction.